Electric logging with oil base drilling fluids



Patented Jan. 20, 1953 'iJNi iiED STATES PATENT QEFECE ELECTRIC LOGGINGWITH OIL EASE DRILLING FLUIDS Application December 12, 1949, Serial No.132,457

i Claims. 1 'I-his invention relates generallyto electric logging withoil base drilling. fluids or muds, and more particularly relates toamethod for rendering certain oil baseidrilling fluids conductive duringelectric logging operations.

The measurement of the resistivity of the different earth formationstraversed by drill holes has-become standardpractice in oil welldrilling during the last twenty years. The techniques in use requirethat. a direct contact be made with the mud filling :the'bore hole bymeans of electrodes'which are electrically connected to the insulatedleadsof the supporting cable. A current of constant intensity isgenerally made to flow in the surrounding medium through one or two ofthese electrodes, called power electrodes. By

ohmic effect potential difierences are produced in resistivity log.

. In certain cases good contact between the electrodes and thesurrounding formation under scrutiny cannot be obtained, such as wherethe -well bore has been drilled with a normally nonconductive oil basedrilling fluid which partially penetrates and seals off the earthformation surrounding the well bore. The oil saturated lining of' thebore hole acts as an electrical insulator, "and electrical contact canonly be established, if at all, by the use of scratchers or otherdevices which must successfully penetrate the oil film and contactthebare earth formation.

A number of other methods such as electromagnetic logging, inductionlogging and the like have been developed to circumvent therequire-"mentor directly contacting the earth formation and the problemsassociated therewith.

These innovations yield data which must be related by empirical methodsto the customary electric log and are disadvantageous for this reason.

The disadvantage of complications in the electric logging does notpreclude the use of oil base drilling fluids since their use offerscertain advantages which considerably outweigh this dis- -advantage. Anyfluid loss from an oil base drillingflui'd is oil and as such isentirely compatible with the oil-bearing formationheaving shales".and'the like. Water base mudson': the otherhand lose-water which oftencauses water blocking-of oilsbearing formations, --wetting *andswelling. of formations such as heavingshales and the like.

The present invention comprises the use of an alternating electricalpotential gradient in conjunction with certain types of oil basedrilling fluids whereby such fluids are rendered electrically conductiveso-that the well bore fluid, as well as the envelope of oil basemud'saturated earth formation cease to be electrical insulators butbecome electrically conductive.

It is an object of this invention to improve the electric logging ofuncased bore holes which have been drilled with, and/or filled with, anormally non-conductive oil base drilling fluid.

It is another object of this invention to render normally non-conductiveoil base drilling fluids electrically conductive.

It is another object of this invention to provide a method and apparatusfor establishing an alternating electrical potential gradient throughouta section of a non-conductiveoil base drilling fluid under suchconditions that the drilling-fluid is made to conduct electrical energy.

It is another object of this invention tocause a dielectric breakdown ofoil base drilling fluids containing both water and metallic soapswhereby such drilling fluids become electrically conductive.

Briefly this invention includes the step of establishin an alternatingelectrical potential gradient throughout the region of an oil'basedrilling fluid whereby the normally non-conductive fluid is renderedconductive.

Only certain oil base drilling fluids have been found which possess thecha'racteristics'of dielectric breakdown and' become more conductiveupon application of an electrical potential gradient of practicalmagnitude. Drilling fluids which display this property and which aretherefore useful in the operation of this invention are those oilbasedrilling fluids which, inaddition to mineral oils and/ordistillat'es thereof, weighing agents and the like, contain more-thanabout 1% water such as between about'l to 20%"by weight of water andmore than-about 11% by weight of a metallic soap such as between about 1and 20% by Weight of metallic-soap. 'Such drilling fluids haveresistiviti'es which customarily are greater than lo 'ohms percentimeter and which are oitengreater' than 10 ohms: per centimeter.

It has been found that where such drilling fluids are. employed inthelfield, they behavesu-bstantially as anhydrous o'il base' dril'lingfluids in that the small amount of "fluidrwhich :isl lost to theformation does not induce water blocking, swelling of clays or any ofthe otherdiniculties 3 generally associated with aqueous drillingfluids. Furthermore, it has been found that where the water content isconsiderably higher, such as 50%, the fluid loss is of such a naturethat it does impair the formation character.

In one modification of the invention two electrode plates, suitablypositioned at different levels in the well bore, are connected to asource of alternating current whereby an electric field is establishedbetween the electrode plates. The equipotential lines passing betweenthe two plates are distorted by the earth formation surrounding the borehole as a result of the conductive character of the bore hole fluidunder the electrical stress.

Two probing electrodes are placed in the electric field between theelectrode plates and in fixed spatial relationship with respect to eachother and to the electrode plates, and when the resulting assembly ismoved coaxially within the bore hole the variation of the potentialdifference between the two probing electrodes is a measure of theelectrical character of the formation surrounding the Well bore. Theprobing electrodes are preferably placed as close to the wall of thewell bore as possible in order that the potential differencetherebetween is most noticeably affected by the changes of theelectrical character of the formation. In the preferred modification theprobing electrodes are pressed against and held next to the formationlining the well bore by a suitable spring mechanism.

The preferred modification of the invention includes the use of twoprobing electrodes disposed between two electrode plates wherein theprobing electrodes are employed to measure potential difierences.However, in another modification of the invention two electrode platesmay be employed without the use of probing electrodes. In the lattermodification the electrode plates in fixed spatial relation to eachother are moved coaxially through the bore hole and the changing powerload required to maintain a particular potential difference between theelectrode plates is then a measure of the changing electrical characterof the surrounding formation.

In another modification of the invention an electrical gradient isestablished between two electrode plates. When an electrode is placed inthe resulting electrical field it establishes a good electricalconnection with the surrounding formation. The electrode forestablishing the electrical connection is preferably pressed or heldagainst the surrounding formation such as by suitable spring mechanism.One or several such electrically contacting electrodes, each suitablydisposed in an electric field between electrode plates, may be used tocarry out any of the conventional electric logging operations such asthe self-potential method. In such a case the contacting electrode inthe electric field merely provides electrical connection with the earthformation through the interposed oil saturated layer which has undergonedielectric breakdown.

In the accompanying drawing, Figure 1 shows the effect of dielectricbreakdown on the resistivity curves for a series of drilling fluids,each of which contains about 2.5% by weight of a soap and each differingfrom the others in the varying percentages of water.

Figure 2 illustrates the fact that in the absence of dielectricbreakdown in the case of two drilling fluids, neither containing morethan a trace of soap, there i no variation of the resis- 4 tivity ofsuch fluids even though potential gradients as high as 1200 volts perinch be em ployed either in the presence or absence of dispersed water.

Figure 3 shows the preferred modification of the invention wherein twoprobing electrodes are mounted between two electrode plates carrying asufficiently high voltage alternating electrical current to cause thedielectric breakdown of the intervening drilling fluid and wherein theelectrical characteristics of the surrounding formation are determinedby the potential difference between the two probing electrodes.

Figure 4 shows another modification of the invention wherein a firstelectrode probe disposed between a first pair of electrode plates isemployed to establish electrical connection with the surroundingformation at a first point and a second electrode probe disposed betweena second pair of electrode plates is employed to establish electricalconnection with the surrounding formation at a second point.

It has been found that certain oil base drilling fluids, namely thosecontaining significant percentages of both water and a metallic soap,exhibit a property which herein is termed dielectric breakdown. Whensuch drilling fluids are subjected to a sufficiently high alternatingvoltage gradient, they cease to be weak dielectrics and becomeelectrical conductors. In the bsence of the dielectric breakdown thefluid exhibits a high resistivity, but when a characteristic voltage isexceeded for the particular fluid, a sharp reduction of the resistivity,often 10 to fold or more, occurs after a short period of application.This invention employs this principle to establish electrical contactbetween one or more contacting electrodes and the surrounding earthformation in those cases where a film or layer of a normallynon-conducting drilling fluid is interposed therebetween. Vfhere thedrilling fluid film or layer is of the aforementioned type, theelectrical contact can be estab lished by subjecting the interposingdrilling fluid to an electrical potential gradient of the propermagnitude whereby the drilling fluid is rendered electricallyconducting.

Drilling fluids which may be employed in this invention are those inwhich a mineral oil is employed as the continuous dispersed phase andamounts to at least about 40% by weight and preferably at least about60% by weight of the fluid. Such drilling fluids may be either of theoil base type or of the emulsion type and may contain small amounts ofcertain additives such as lime, bentonite, barytes, starch and the like.In order that the drilling fluid to operable in this invention it mustcontain both water and a metallic soap such as between about 1 and 20%by Weight of water and preferably between 7 and 17% by weight of waterand between about 1 and 20% by weight of a metallic soap and preferablybetween about 2 and 8% by weight of a metallic soap. The metallic soapof the drilling fluids employed in this invention is preferably anionizable soap such as an alkali or alkali metal soap derived fromhigher molecular weight carboxylic or sulfonic acids such as those acidswhich contain between about 2 and 35 carbon atoms per molecule andpreferably between about 10 and 25 carbon atoms per molecule. Among theacids which may. be employed as their metallic soaps are variousnaphthenic acids, rosin acids, modified rosin acids obtained by heattreatment of rosin acids with or without the use of catalysts, such asby heatnotedin all cases with thesingular ing; at"250.?- -35 0- C. untilthe specificretationis increased-from a negative value to +5 to +15,

fatty acids such a s'stearic acid, and olefinic acids such as oleic acidand the like. Sulfonic acids which are suitable for use in thisinvention may be conveniently prepared by the sulfonation of petroleumstocks with sulfuric acid, fuming sulfuric acid, chloro-sulfonic acidand the like. In' the sulfonation of lube oil stock to prepare a whiteoil, the sludge is separated and the oil isthen extracted with sodiumhydroxide whereupon the sodium soap of the sulfonic acid is obtained,which issuitable for use in this invention.

Referring now more particularly to Figure 1, this figureshowsthe effectof variations of the percentage of waterin a drilling fiuid onres'istivity changes caused by th dielectric'breakdown of the fluid. Adrilling fluid stock'was-prepared by: intimately mixingtogether thefollowing ingredients wherein themetallic soap was the potassium soapderived from; aheat-mod-ifled abietic acid.

Table I Ingredient: Percentage. byweight Po assium. soap 2.5 Bentonite1.25

. ""T Petroleum distillate (400-800 F. bQiling range) 10.0 Light fuel011-- 85.5

Total 100.00

The modified abietic acid-was prepared :by heatingabietic-acid-at280-.-320 C..for about e hours. The foregoingstock wasthen mixed with sufficient water to form-a series of mixtures containing0%, 2%, 7%,,12%.and' 17% of water respectively.

The dielectric breakdown of each. of the fluid mixtures was measured inthe following'manner. Two inch platinized steel rods about12 inches inlength were immersed in the fluid mixture and maintained in parallelrelationship about T33 of an inch. apart. A variable source of 60 cyclealternating voltage connected to the tworods was used to establishpotential gradients in excess of 250 volts per inch in the fluid mixturebetweenthe rods. Ineach case the instantaneous resistivityv of theparticular fluid :was calculated from the instantaneous measurements ofcurrent flow and the voltage.

Figure 1 shows that all, of the-fluids containing water in addition toth metallic soap show a characteristic decline in the resistivity withtime when the potentialgradient is 250 volts per inch, with eachresistivity ultimately arriving at a minimum-which-is progressivelylower for the higher percentages of water. In the absence of water thefluid exhibits no decrease in resistivity evenunder a potential gradientof 2.50volts per inch.

When the. same series of measurements isr-epeated using potentialgradient; of only 160 volts per inchbetween the rods, as im1lar decreaseof resistivity with increasing exposure tim is exception of the samplecontaining no water. However, whenv the. measurements are repeated usinapotential gradient of. only '5 volts per inch be ween fluids show aconstant high does? not decrease with increasresistivity which matim-ofappl tion q r n obtained whe h Analogous -results are tallio soap isthe sodium soap of a mahogany acid" obtained inthe manufactureofwhiteoil. In;such a case the lube oil stock is sulfonated with fumingsulfuricacid and the sludge is separated therefrom. The acid containingoil is then neutralized with sodium hydroxide whereupon the scraps areobtained. 'Such soaps may be employed directly without separation fromthe residual oil if desired.

When potassium oleate is employed as the soap, a similar result is alsoobtained. Other alkali and alkali metal soaps of sulfonic and carboxylicacids having between 2 and 35 carbon atoms per molecule behave 1similarly.

Referring now more particularly to Figure 2, this. figure illustratesthe absence of dielectric breakdown in oil basedrillingfluids which arefree of, or substantially lacking in, metallic soaps. A first sample ofdrilling fluid containingabout 0.8% by Weight of water was preparedbymixing the following ingredients.

Table 11 Ingredients: Percentage by weight :Asphalt 15Petroleum'distillate' 01001-800915. boiling range.) n 7.6 Water 0.8Lime. 4 Calcium carbonate (weighting agent) 4.2

Total no.0

A second sample was prepared-by mixing an aliquot of the first samplewith sufficientwater to form. an intimate, mixture containing about 10%,by Weight of water. While no metallic soap was added directly to eitherof the two samples, nonetheless the natural acid normally present in,the, asphalt, and petroleum distillate werev sufficient to react withthe lime and form about 0.3 by weight. of calcium soaps.

The results obtained by repeating the resistivity measurements describedhereinbefore, wherein. the two preceding samples were measured atpotential gradientsup to 1200 volts per There was no change inresistivity with. time or with magnitude of potential gradient,. thatis, nobreakdown volt-' ages were observed.

Referring now more particularly to Figure 3,

bore-hole ll has been. completed. through various earth formations 12with the use of an oil base drilling.-fiu-idcontaining. both water and ametallic soap-ofthe type described hereinbefore. Bore hole vH remainsfilled with a. drilling fluid l3 and a part of the fluid has penetratedportions of: the wall of the bore hole thereby electrically insulating;such. portion of the bor holegwall. At

:thaeerth. ur a e. pulley 4.0. supports cabled] .the

u sreed of; which is woundpn Winding drum 4 Winding drum. 42 isfittedwith depth of instrument indicating means not shown. Winding drum4,2 is also fitted with suitable contactors 43 which may take'the formof slip rings. Leads 44 and 45 connect alternating, current generator 46to-contactors 4 3, which in turn, are connected to power leads 4'! and48 carried within cable 4|. .Cable 4 lrunsdownwardly from the earthsurface through. bore. hole H and supports instrument housing l6whichcomprises a sealed container housing a high voltage step-uptransformer H an alt nat n potentia recordin m n The. lower portion. ose ledl..inst um nth sin l 6 is, fitted with electrode plate l9towhichis-attached; a hollow shaftZil which in turnsupports lowerelectrode plate 2 l' parallel toand at a fixed distance from the firstelectrode plate I9. Nonconducting flexible supporting members 22 and 23are suitably mounted in the inner space between parallel electrodeplates |9 and 2|. Each non-conducting supporting member mounts a pair ofprobing electrodes or probes, probes 24 and 25 being mounted on member22 and probes 26 and 21 being mounted on member 23 respectively. Probes24 and 25 bear the same vertical distance relationship to each other andto parallel electrode plates l9 and 2| respectively as do probes 26 and21 respectively. Upper probes 24 and 2B are interconnected by electricallead 28 while lower probes 25 and 21 respectively are interconnected bylead 29. Leads 28 and 29 pass into hollow shaft 20 through suitableinsulators and thence upwardly through shaft 29 to alternating potentialrecording means IS.

The primary of transformer H is connected to leads 4'! and 48 which arein turn connected to alternating current generator 45 in the mannerdescribed hereinbefore. The secondary of transformer I1 is connected byelectrical lead 30 to electrode plate l9 and by lead 3 I, passingthrough hollow shaft 20, to lower electrode plate 2|.

In the operation of the equipment the instrument is lowered in the borehole by means of supporting power supply cable 4| to the approximatelevel of the formation to be logged. An alternating current istransmitted from alternating current generator 46 by connecting leadsand commutators to transformer I! which steps up the voltage as desired.The output of transformer ll is supplied to the two electrode plates I9and 2| respectively whereby an electric field is set up in the oil basedrilling fiuid disposed between and around the electrode plates. As aresult of high electrical stress the surrounding drilling fluidundergoes a dielectric breakdown and becomes electrically conductive.The lines of electrical force between the two electrode plates l9 and 2|extend outwardly through the conducting oil and the oil film into theformation. Accordingly, the conductivity of the surrounding formationaffects the electric field set up by the electrode plates. As theapparatus is raised and lowered in the bore hole, the changingelectrical character of the formation beyond the permeated oil layeralters the electrical field between the electrode plates, and therebychanges the voltage gradient existing between upper probes 24 and 26 andlower probes 25 and 21 respectively. The changing electrical potentialbetween the two pairs of probes as the apparatus is moved through thelength of the bore hole is transmitted through connecting leads 28 and29 to alternating voltage potential recording means I8. Voltagerecording means I8 is any convenient type of instrument for measuringand recording alternating voltage diiferences. One convenient form ofsuch instrument is a voltmeter of the conventional type wherein a lightis caused to shine on a mirror attached to the pointer of the voltmeter.The swing of the voltmeter causes the reflected light beam to swing backand forth across the moving strip of photographic film whereby voltagevariations produce corresponding fluctuations of the position of thereflected light beam on the photographic film. Development of thephotographic film gives a record of the measurements. When themeasurements are correlated with the depth, there is obtained anelectrical record of the character of the formation surrounding the borehole in any location.

For the convenience of stepping up the voltage the frequency of theelectrical current supplied from alternating current generator 46 ispreferably between about 25 and 400 cycles per second. The use offrequencies of less than about 25 cycles per second often leads toadverse polarization effects. The voltage produced by alternatingcurrent generator 46 is any suitable voltage which can be transmittedthrough cable 4| without severe losses and insulating difiiculties. Forexample. a suitable voltage output would be in the range between about40 and 1000 volts. Transformer I1 is adapted to step up the supplyvoltage from alternating current generator 46 to the required voltage toestablish a dielectric breakdown of the oil base mud between electrodeplates 9 and 2| respectively. The use of transformer simplifies somewhatthe transmission of the electrical energy down the bore hole byeliminating the necessity of energy transmission at higher voltages withattendant insulation problems.

It is generally necessary to establish a voltage gradient of about voltsper inch and preferably about 200 volts or more per inch in the oil basedrilling fluids of the invention in order to effect their dielectricbreakdown. The physical separation between the opposing exposed faces ofthe electrode plates I 9 and 2| respectively is normally between about 1and 20 inches and preferably between about 1 and 8 inches. The output ofthe transformer I1 is preferably in the range of 1000 to 3000 volts.

While the foregoing description of the invention has to some extent beenlimited to those frequencies which are conveniently stepped up by thetransformer, it is apparent that higher or lower frequencies may beemployed if suitable means for their generation, such as an electricoscillator and amplifier, is included in the apparatus. When suchgenerating means are employed, they may be located in the bore hole andnear the electrode plates whereby the necessity of extensive insulatingmeans for the transmission of high voltage electric currents iseliminated.

In the foregoing description of this invention two upper probes, 24 and25 respectively, and two lower probes, 25 and 21 respectively have beenemployed. Upper probes 24 and 26 are electrically connected in paralleland lower probes 25 and 21 similarly are electrically connected inparallel. It is apparent that the invention is not limited to the use ofonly two upper and two lower probes. In some cases only a single upperand a single lower probe may be employed, and in other cases multiplewires or even wire mesh may be used to make the resulting log morenearly representative of the entire formation surrounding the unit.

Although the foregoing description of this inventionhas been directedprimarily to the continuous application of electrical current to theelectrode plates, short bursts of electrical energy may be employed forcertain specific cases. Thus, after the minimum resistivity is obtainedby exposure to the proper potential gradient for the required length oftime, e. g. 25 to 500 seconds, the electrical current is applied by asuitable timing device to the electrode plates for a short period oftime, such as for example 10 to 500 milliseconds. The activation voltageis thereupon disconnected for a short period of time, such as 10 to 500'milliseconds. during which time electrical'measurements of thesurrounding formation are made. Follow- 9 ing the measuring period thenext activation period is commenced with the sequence of activating andmeasuring being repeated numerous times as desired.

With certain drilling fluids of this invention it is possible toestablish the breakdown of the oil with a relatively high voltage andthereafter maintain the breakdown by the continuous application of alesser voltage provided a characteristic minimum is maintained. When thevoltage falls below the minimum characteristic for the particular fluid,the current fiow drops rapidly to a very low value "as the resistivityincreases.

In another method for electrically logging a formation, the equipmentshown in Figure 3 is operated by simply supplying an alternating currentfrom transformer H to electrode plates l9 and 21 and measuring thechanges in power consumption as the apparatus is moved through the borehole. In such a case the previously described probing electrodes, 24 and26, and 25 and 2! may be utilized simultaneously as describedhereinbefore or they may be dispensed with entirely and a single log ofthe power consumption may be obtained.

Referring now more particularly to Figure 4, bore hole i has beencompleted through various earth formations 52, the drilling operationhaving been carried-out with an oil base drilling fluid containing botha metallic'soap and water. Bore hole 51 remains partially filled withsuch oil base drilling fluid 53. Sealed housing 54 is suspended withinbore hole 51 below the surface of drilling fluid 53 by means ofsupporting cable 55 which also carries insulated power leads 5? and 58,respectively. At the'earth surface power leads 5'! and 58 areelectrically connected to a suitable source of alternating electricalcurrent by a suitable means not shown. Sealed housing 56. containstransformer 59 which is equipped with a tapped secondary winding andalso a potential measuring and recording means lit.

Attached to the lower end of housing 5% is first upper-electrode plate6|. Hollow non-conducting supporting shaft 62 extends downwardly fromits point of attachment to electrode plate 6!. Lower first electrodeplate 63 is supported by supporting shaft 62 a short distance belowupper first electrodeplate Si in parallel relationship thereto.Flexible'non-conducting supportingrnembers E4 and 65respectivelyare-suitably mounted between upper and lower first electrodeplates 'SI and 63 respectively. Each of the two supporting members 64and 65 respectively mounts probing electrodes Bfi-andt'i respectively,both electrodes being electrically connected in parallel by means oflead 68. Some distance below the lower first electrode plate 63 an uppersecond electrode platefiil and a lower second electrode plate aremounted on supporting shait 62 in parallel relationship'to'each otherand to the upper and lower first electrode plates iii and 63respectively. In a manner similar to that described hereinbefore forupper and lower first electrodes plates '6! and '63 "respectively; upperand lower "second electrodefplates t9 and it furnish support fortwo-flexible non-conducting supporting members 'l'l and1'2 respectively.The two supporting'members 'H and IZ'inturn'mount probingelectrodes'13,and respectively, which probing electrodes are adapted topress against a part of the formationin betweenthe two second electrodeplates. Probing electrodesla and M areelectricallyconnected by lead 15.

The first outer terminal of the secondary of transformer 59 is connectedby electrical lead 76 to upper first electrode plate SE. The middletapped output of transformer 59 is connected by lead Ti to lower firstelectrode plate E3 and to lower second electrode plate it. The secondouter terminal of transformer 59 is connected by means of lead 18 to theupper second electrode plate 69.

Potential measurin and recording instrument 553, which is preferably ofthe photographic type described hereinbefore, is connected through lead68 to probing electrodes 66 and 61 respectively, and is connectedthrough lead E5 to probing elec trodes i3 and M respectively.

In the operation of the apparatus of Figured an alternating electricalcurrent of the type described hereinbefore is supplied through leads 5?and 53 to transformer 5% which suitably steps up the voltage. Thevoltage difference between the first outer terminal and the middleterminal is employed to establish a potential gradient between the upperand lower first electrode plates 61 and 63'respectively, therebyproviding for the dielectric breakdown of the drilling fluid within theresulting first electrical field. The voltage difference between themiddle terminal and the second outer terminal is employed to establish apotential gradient between the upper andlower second electrode plates 69and 19 respectively, thereby providing for the dielectric breakdown ofthe drillin fluid within the resulting second electric field. Theelectrical conducting character of the formation permit varying flow ofelectrical current between the first electrical field and the secondelectrical field owing to the potential difference between theinstantaneous potentials of the two electric fields. The variations ofcurrent flow are indicated by the corresponding variations in thepotential difference between the first pair of probin electrodes and thesecond pair of probing electrodes. This latter potential difierence istransmitted through leads 68 and I5, respectively, to potentialmeasuring and recording means 86. Therecording means preferably operatesin the-manner described hereinbefore in connection with Figure 3although other suitable'means may be employed. Development of thephotographic film gives a measure of the electrical character of theformation surrounding the bore hole between the first and secondelectrical fields.

Although transformers disposed within the bore hole have been employedto illustrate the principles of this invention, it is apparent that theuse of such transformers is not a requirement of the invention. Thus, ahigher voltage source at the earth surface may be employed with suitableprovision being made to transmit such voltage down the bore hole'to theelectrode plates.

It is "apparent that the invention is not limited to the use of only twoprobe electrodes connected in parallel and mounted in between each pairof electrode plates. One or several such probingelectro-des may beemployed.

In the foregoin description of the particular modification of theinvention, the high voltage leads 16 and F1 to electrode plates 6| and63 respectively maybe reversed with or without the simultaneous reversalof the high voltage leads H and 18 to electrode plates 69*and HIrespectively, or the latter voltage leads 1 I and it alone may bereversed.

It is apparent that in the general application of theinventiononeormoreelectricfields are set up between one or more pairs ofelectrode plates,

which pairs of electrode plates may be separated from other pairs ofelectrode plates by any desired distance, which plates may or may not beplanar and which plates are in electrical contact with the surroundingoil base drilling fluid. The probing electrodes are disposed within eachof the electric fields and are employed to establish electrical contactbetween voltage or current measuring apparatus and the earth formationsurrounding the electric field or alternatively to provide an electricconnection for impressing a voltage upon the formation. By using thisgeneral method various types of logs may be made in conventional manner,e. g., by the self-potential method wherein a first contact electrode ismoved up and down the bore hole between two electrode plates and thesecond contact electrode is embedded in and located in the earthsurface.

In another modification of the invention the earth formation itself maybe made to serve as one of the electrode plates. Two electrodes areplaced in the bore hole and are separated by a known distance. A centertapped source of alternating current is employed and the one outerterminal is electrically connected to the one electrode while the otherouter terminal is connected to the other electrode. The center tap isgrounded such as at the earth surface. Under these conditions the earthformation is always at a different potential than either of the twoelectrodes. When this potential difference is sufflcient to cause adielectric breakdown of the oil base mud between electrode and thesurrounding formation, electric current begins to flow through theformation between the two electrodes. The magnitude of the current isdependent upon the electrical character of the surrounding formation andis most easily determined from the power consumed in maintaining theconstant potential between the electrodes. For such purpose asufficiently high voltage is employed to cause the dielectric breakdownof the oil between the formation and the electrodes.

In the general application of this invention there is a time delaybetween the application of the activating voltage and the dielectricbreakdown. Once the breakdown has been established, such as after about25 to 500 seconds of application of the breakdown voltage, only shortbursts of activating energy are required to sustain the loweredresistivity of the drilling fluid. Thus. after the initial activating,short bursts of electrical energy of about to 500 milliseconds durationmay be employed after short periods of measurement taking, which may beof about 10 to 500 milliseconds duration.

It is apparent that many modifications of this invention may be made bythose skilled in the art without departing from the spirit and scope ofthe following claims.

I claim;

1. A method for electrically contacting a portion Of a bore hole whereinsaid portion of said bore hole has been drilled with a drilling fluidwhich comprises at least about 60% by weight of mineral oils anddistillates thereof, between about 1% and about by weight of water, andbetween about 1% and about 20% by weight of a metallic soap selectedfrom the class consisting of alkali and alkali metal soaps of carboxylicand sulfonic acids having between about 2 and carbon atoms per molecule,which method of contacting comprises establishing a voltage gradientzone throughout a vertical portion of said bore hole, said voltagegradient of said voltage gradient zone being at least about volts perinch, thereby efiecting the dielectric breakdown of said drilling fluidin said zone, and passing electrical energy from an electrode disposedin said zone to the section of the wall of said well bore surroundingsaid zone and thereby establishing electrical contact therewith.

2. A method of electrically logging a portion of a bore hole whereinsaid portion of said bore hole has been drilled with a drilling fluidwhich comprises at least about 60% by weight of mineral oils anddistillates thereof, between about 1% and about 20% by weight of water,and between about 1% and about 20% by weight of a metallic soap selectedfrom the class consisting of alkali and alkali metal soaps of carboxylicand sulfonic acids having between about 2 and 35 carbon atoms permolecule, which method of logging comprises establishing a voltagegradient zone throughout a vertical portion of said bore hole, saidvoltage gradient of said voltage gradient zone being at least aboutvolts per inch, thereby effecting the dielectric breakdown of saiddrilling fluid in said zone, moving said voltage gradient zonevertically within said bore hole and measuring the voltage differencebetween a point within said zone and a portion of said oil bearingformation.

3. A method of electrically logging a portion of a bore hole whereinsaid portion of said bore hole has been drilled with a drilling fluidwhich comprises at least about 60% by weight of mineral oils anddistillates thereof, between about 1% and about 20% by weight of water,and between about 1% and about 20% by weight of a metallic soap selectedfrom the class consisting of alkali and alkali metal soaps of carboxylicand sulfonic acids having between about 2 and 35 carbon atoms permolecule, which method of loggin comprises establishing a voltagegradient zone throughout a vertical portion of said bore hole, saidvoltage gradient of said voltage gradient zone being at least about 100volts per inch, thereby efiecting the dielectric breakdown of saiddrilling fluid in said zone, moving said zone vertically within saidbore hole and continuously measuring the electrical energy required tosustain said voltage gradient.

4. The method of electrically contacting a well bore wherein said wellbore has been drilled with a drilling fluid which comprises at leastabout 60% by weight of mineral oils and distillates thereof, betweenabout 7 and 17% by weight of water, and between about 2 and 8% by weightof a metallic soap selected from the class consisting of alkali andalkali metal soaps of carboxylic and sulfonic acids having between about2 and 35 carbon atoms per molecule, which method of contacting comprisesintroducing two spaced electrode plates below the surface of said oilbase drilling fluid in said well .bore, supplying an alternating currentto said two electrode plates having a frequency between about 25 and 400cycles per second and thereby establishing a voltage gradient in thesaid drilling fluid between said two electrode plates of at least about100 volts per inch, and passing electrical energy between a probingelectrode, maintained in fixed spatial relationship to said twoelectrode plates, and the wall of the well bore laterally surroundingsaid probing electrode and thereby establishing electrical contacttherewith.

5. The method of electrically contacting a well bore wherein said wellbore has been drilled with a drilling fluid which comprises at leastabout 60% by weight or mineral oils and distillates thereof, betweenabout "I and 17% by weight of water, and between about 2 and 8'%byweight of a metallic soap selected from the class consisting of alkaliand alkali metal soaps of carboXylic and sulfonic acids having betweenabout 2 'and 35 carbon atoms per molecule, which method comprisesintroducing two spaced electrode plates below thesurface of said oilbase drilling fluid in said well bore, supplying an alternating currentto said two electrode plates having a irequencybetweenabout 25 and 400cycles "per secnd and" thereby "establishinga voltage gradient in'thesaid drilling fiui'd between 'said two "electrode plates "of at leastabout 100 vans per inch, continuing said supplying of alternatingcurrent for a period of between about 25 seconds and 500 seconds,discontinuing said supplying of said alternating current during ameasuring period of between about and 500 milliseconds, continuing saidsupplying of said alternating current during an activation period ofbetween about 10 and 500 milliseconds, and repeating said sequence ofmeasuring periods and activation periods, and passing an electricalcurrent between a probing electrode maintained in fixed spatialrelationship to said two electrode plates and the wall of the well borelaterally surrounding said probing electrode during a measuring period,and thereby establishing electrical contact therewith.

6. The method of electrically logging a well bore wherein said well borehas been drilled with a drilling fluid which comprises at least about60% by weight of mineral oils and distillates thereof, between about 7and 17% by weight oi water, and between about 2 and 8% by weight of ametallic soap selected from the class consisting of alkali and alkalimetal soaps of carboxylic and sulfonic acids having between about 2 and35 carbon atoms per molecule, which method comprises introducing twospaced electrode plates below the surface of said oil base drillingfluid in said bore hole, establishing an alternating current voltagegradient of at least about 100 volts per inch and of a frequency betweenabout and 400 cycles per second in said drilling fluid between said twoelectrode plates, thereby effecting the dielectric breakdown of said oilbase drilling fluid near and between said two electrode plates, andmeasuring the potential difference between two probing electrodes whichare maintained in fixed spatial relationship to said two electrodeplates and to each other.

7. The method of electrically logging a well bore wherein said well borehas been drilled with a drilling fluid which comprises at least about60% by weight of mineral oils and distillates thereof, between about 7and 17% by weight of water, and between about 2 and 8% by weight of ametallic soap selected from the class consisting of alkali and alkalimetal soaps of carboxylic and sulfonic acids having between about 2 and35 carbon atoms per molecule, which method comprises introducing aseries of four electrode plates into said bore hole below the surface ofsaid oil base drilling fluid, said series comprising a first electrodeplate, a second electrode plate disposed below said first electrodeplate, a third electrode plate disposed below said second electrodeplate and a fourth electrode plate disposed below said third electrodeplate, supplying a first alternating current voltage to said first andsaid second electrode plates and establishing a voltage gradient of atleast about 100 volts per inch and causing the dielectric breakdown ofthe portion of said oil base drilling fiuid 'd-isposed near "and "therebetween, supplying a *second alternating current voltage to said thirdand -said four'th electrode plates and thereby -establishing avoltagegradient of at least volts perinchand causing the dielectric breakdownofthe portion of sai'z ibil base drilling fluid disposed nea-r and therebetween, and measuring the voltage difference between a first probingelectrode maintained in fixed spatial relationship 'to said firstelectrode plate and said second electrode-plate, and a-secand probingelectrode maintain'edin fixed-spatial relationship to saidthirdelectrode pl a'te and said fourth electrode 1 plate.

'8'. An apparatus for' electrically logging a-well bore which comprisesat =least about 60% by weight of mineral oils and distillates thereof,between about 1% and about 20% by weight :of water, and between about 1%and about 20 by weight "of a metallic soap selectedf-rom the classconsisting "of alkali amal am metal soapscf carboxylic andsulfonic"acids-having between about 2 and 35 carbon atoms per molecule,a unit adapted for coaxial movement within a well bore which comprisesan upper electrode plate, a lower electrode plate, a probing electrodedisposed therebetween, an alternating current generating means connectedto said upper and said lower electrode plates, said alternating currentgenerating means having a generating capacity greater than about 100volts per inch of separation distance between said upper electrode plateand said lower electrode plate, and electrical measuring means connectedto said probing electrode for determining the difference of potentialbetween said probing electrode and a portion of the oil bearingformation surrounding said well bore.

9. An apparatus for electrically logging a well bore which comprises atleast about 60% by weight of mineral oils and distillates thereof,between about 1% and about 20% by weight of water, and between about 1%and about 20% by weight of a metallic soap selected from the classconsisting of alkali and alkali metal soaps of carboxylic and sulfonicacids having between about 2 and 35 carbon atoms per molecule, a unitadapted for coaxial movement within a well bore which comprises an upperelectrode plate, a lower electrode plate, two probing electrodesdisposed between and maintained in fixed vertical relationship to saidupper electrode plate and said lower electrode plate, an alternatingcurrent generating means connected to said lower electrode plate andsaid upper electrode plate, the generating capacity of said alternatingcurrent generating means being greater than 100 volts per inch ofseparation distance between said lower electrode plate and said upperelectrode plate, and an electrical measuring means connected to said twoprobing electrodes for measuring the potential difference therebetween.

10. An apparatus for electrically logging a well bore which comprises atleast about 60% by weight of mineral oils and distillates thereof,between about 1% and about 20% by weight of water, and between about 1%and about 20% by weight of a metallic soap selected from the classconsisting of alkali and alkali metal soaps of carboxylic and sulfonicacids having between about 2 and 35 carbon atoms per molecule, a unitadapted for coaxial movement within a well bore which comprises an upperelectrode plate, a lower electrode plate, a series of four electrodeplates which comprises a first electrode plate, a

15 second electrode plate disposed below said first electrode plate, athird electrode plate disposed below said second electrode plate and afourth electrode plate disposed below said third electrode plate, afirst probing electrode disposed between said first and said secondelectrode plates, a second probing electrode disposed between said thirdand said fourth electrode plates, a first alternating current generatingmeans connected to said first and said second electrode plates, thegenerating capacity of said first alternating current generating meansbeing greater than 100 volts per inch of separation distance betweensaid first and said second electrode plates, at second alternatingcurrent generating means connected to said third and said fourthelectrode plates, the generating capacity of said second alternatingcurrent generating means being greater than 100 volts per inch ofseparation distance between said third and said fourth electrode plates,and means for determining the potential difference between said firstprobing electrode and said second probing electrode.

ROLAND F. KRUEGER.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number UNITED STATES PATENTS OTHER REFERENCES Sawdon-Electrical loggingin oil base drilling fluid-Petroleum Engineer-pages 124, 126-JuneMicrolog-Doll-Oil 8: Gas Journal, March 2,

1950-pages 62, 64, 69.

1. A METHOD FOR ELECTRICALLY CONTACTING A PORTION OF A BORE HOLE WHEREINSAID PORTION OF SAID BORE HOLE HAS BEEN DRILLED WITH A DRILLING FLUIDWHICH COMPRISES AT LEAST ABOUT 60% BY WEIGHT OF MINERAL OILS ANDDISTILLATES THEREOF, BETWEEN ABOUT 1% AND ABOUT 20% BY WEIGHT OF WATER,AND BETWEEN ABOUT 1% AND ABOUT 20% BY WEIGHT OF A METALLIC SOAP SELECTEDFROM THE CLASS CONSISTING OF ALKALI AND ALKALI METAL SOAPS OF CARBOXYLICAND SULFONIC ACIDS HAVING BETWEEN ABOUT 2 AND 35 CARBON ATOMS PERMOLECULE, WHICH METHOD OF CONTACTING COMPRISING ESTABLISHING A VOLTAGEGRADIENT ZONE THROUGHOUT A VERTICAL PORTION OF SAID BORE HOLE, SAIDVOLTAGE GRADIENT OF SAID VOLTAGE GRADIENT ZONE BEING AT LEAST 100 VOLTSPER INCH, THEREBY EFFECTING THE DIELECTRIC BREAKDOWN